The control of ink feed in a continuous printing process is an effective means for improving the quality of a printed image. With ink feed control it is the aim to achieve a high degree of conformity between the target colors of a printing copy, e.g., an "o.k. sheet" printed in the machine, an original, a proof, or in some instances, printing plates used for applying individual process colors, and the colors of a printed product from a production run.
Spectral measurements of emissions from color measuring fields or color bars, the mathematical conversion of these measured values into colormetric values, and further into control data for adjustment of ink feed elements of a printing press have become known from European Patent No. 0 228 347. To conform or color match a printing copy and a printed product, the spectral emissions of color measuring fields or color bars from the printing copy and the printed product are measured. From the measured emissions, the color coordinates of a reference color spot on the printing copy and the respective color coordinates of an actual color spot on the printed product are determined. Through a comparison of the emissions and the color coordinates of the reference color spot with the respective emissions and color coordinates of the actual color spot, the color difference between the reference color spot and the actual color spot is determined. This color difference is converted into change values for layer thicknesses of individual printing inks. The control of the ink feed elements themselves takes place in accordance with the determined change values of layer thicknesses of the individual printing inks so that the total color difference between the reference color spot and the actual color spot becomes minimal.
Japanese Patent No. 2-32566 is directed to a device for determining dot area coverages of colored printed products. Screen densities of measuring points on the colored printed products are measured through red, green, and blue filters. From the measured screen densities the area coverages in the process colors cyan, magenta, and yellow are determined by the Murray-Davis formula. The theoretical screen densities are then determined from the area coverages by the Yule-Nielsen formula. The theoretical screen densities are compared with the measured screen densities by an iterative method. The screen densities and area coverages are adjusted so that a deviation between the theoretical screen density and the measured screen density lies within a given tolerance.
The device disclosed in Japanese Patent No. 2-32566 is limited, however, to the three standard printing inks, cyan, magenta, and yellow. A color control of the color black or special colors of printing ink is not provided for with the method described therein.